Mutant selection techniques, that will permit the isolation of both negative and altered regulatory mutants for several key enzymes present in Streptococcus mutans, have been developed. Using an enzymatic detection technique, we have successfully isolated two mutants of S. mutans that are missing the enzyme glutamate dehydrogenase, which is capable of fixing free ammonia for the synthesis of amino acids. Isolation of these mutants will permit us to determine the mechanism of ammonia assimilation and its contribution to the successful colonization of the oral environment by S. mutans.